Fuel cells are considered a possible alternative to direct combustion engines to power transportation vehicles and to possibly furnish electrical energy to a power distribution grid for home and business use. In a fuel cell, fuels are chemically reacted with an oxidant whereby a direct current is produced at a low voltage across individual cells and stacks of cells produce useful power. Catalyst materials promote the chemical reactions of the fuels (typically hydrogen or methanol) and oxidant (typically pure oxygen or air).
In a generic embodiment shown in FIG. 1, a fuel cell 10 includes an anode electrode 14 for the fuel oxidation, a cathode electrode 16 for the oxidant reduction, and a solid-state polymer electrolyte membrane 18 therebetween to provide an ionic conduction path. The combination of anode electrode 14, cathode electrode 16, and membrane 18 is conventionally called a membrane electrode assembly (MEA) 12. A suitable catalyst is disposed adjacent the interfaces of electrode/membrane surfaces 14/18 and 16/18 so that the fuel is oxidized at the anode/membrane interface 14/18 to produce ions that traverse the membrane to complete the oxidant reduction at the cathode/membrane interface 16/18. Fuel 24 is distributed over anode 14 of MEA 12 by fuel distribution plate 22 and unreacted fuel and reaction products 26 are exhausted. Oxidant 32 is distributed over cathode 16 of MEA 12 by oxidant distribution plate 28 and excess oxidant and reaction products 34 are exhausted. As a result, electrons generated at anode 14 travel through an external circuit (not shown) back to cathode 16. The electrons constitute the flow of electrical current that provides energy to components connected to the external circuit.
The most common fuel used in the development of polymer electrolyte membrane fuel cells has been hydrogen, either in a pure form or furnished as a reformate from hydrocarbon products. Yet another approach is to directly use a liquid methanol solution in direct methanol fuel cells (DMFCs) to avoid the complications associated with supplying pure hydrogen or providing a separate system for reforming hydrocarbons to provide reformed hydrogen. DMFCs with a solid polymer electrolyte can provide high current density at low temperature and have a relatively simple fuel cell construction. Methanol is a renewable fuel material and can be readily transported and supplied with existing transportation and distribution infrastructure for liquid fuels. Both hydrogen fuel cells and DMFCs have the generic structure shown in FIG. 1.
In a DMFC, catalysts promote electrode reactions at the cathode and the anode, where a metric of performance is the catalytic activity per unit mass of catalytic metal. This metric is directly related to the efficiency and output power of the cells and to the manufactured cost of the cells. Platinum black was an early cathode catalyst in an ion-exchange MEA for hydrogen fuel cells, typically a gas diffusion electrode substrate with one surface coated with platinum black in an amount of 4 to 10 mg cm−2 of the MEA.
To improve the utilization efficiency of platinum, a catalyst was developed with a platinum alloy supported on conductive carbon or an unsupported platinum alloy, which was mixed with an ion-exchange polymer, coated on an electrode substrate, and joined to an ion-exchange membrane by painting, hot pressing, or the like, to form the MEA. This process permitted the thickness and composition of the catalyst layer to be controlled so that the catalyst was more effectively utilized in the electrode reaction. A loading of platinum or platinum alloy of only 0.1 to 1.0 mg cm−2 was needed to produce a performance equivalent to prior art hydrogen fuel cells.
This reduced loading that has been demonstrated for hydrogen fuel cells has not been achieved for DMFCs. In a DMFC, some methanol crosses through the membrane from the anode and reacts at the cathode, competing with the oxygen reduction reaction for active catalyst surface sites. Reducing the catalyst loading results in fewer active sites available for the oxygen reduction reaction, as well as limiting the ability of the catalyst to handle methanol crossover, with a concomitant reduction in the potential of the DMFC cathode. Thus, methanol crossover to the cathode not only lowers fuel utilization, but also adversely affects the oxygen cathode with overall lower cell performance.
One way to reduce the effect of methanol crossover on DMFC performance is to simply reduce methanol crossover by developing a membrane that is less permeable to methanol. However, this has not yet been fully realized in the art. Other ways to reduce methanol crossover include lower methanol feed concentration and optimized cell design.
The present invention recognizes that performance losses associated with methanol crossover arise from the fact that most Pt-based cathode systems are catalytically active to methanol oxidation under normal cell operating conditions with a resulting net cathode potential from oxygen reduction reaction potential reduced by the methanol oxidation reaction. In accordance with the present invention, a Pt-alloy catalyst has been identified that is less catalytically active for methanol oxidation, while having equal or increased catalytic activity for oxygen reduction.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.